1. Field of the Invention
This invention concerns a method for recovering viscous petroleum from a subterranean, viscous petroleum-containing formation in which the mineral content of the formation is substantially unconsolidated, such as the unconsolidated tar sand deposits.
2. Description of the Prior Art
There are many subterranean petroleum-containing formations in various parts of the world from which petroleum cannot be recovered by conventional means because the petroleum is too viscous to flow or be pumped. The most extreme example of such viscous petroleum-containing formations are the so called tar sand or bituminous sand deposits such as those located in numerous western states in the United States and in Alberta, Canada, as well as in Venezuela. Other smaller deposits exist in Europe and Asia.
Tar sands are generally defined as sand saturated with a highly viscous crude petroleum material not recoverable in its natural state through a well by ordinary production methods. The petroleum constituent of tar sand deposits is highly bituminous in character and the viscosity at normal formation temperatures of about 50.degree. F is in the range of a million centistokes. While this is a very high viscosity, the viscosity-temperature relationship is exceedingly sharp, and the viscosity drops to about 20 centistokes at a temperature of about 300.degree. F. The sand present in tar sand deposits is generally fine quartz sand, in many cases waterwet and the bituminous petroleum material occupies most of the void space around the water-wet sand grains. The balance of the void space is filled with connate water, with some deposits containing small volumes of gas such as air or methane. Even in those formations in which the sand grains are in contact, the void volume of the formation is about 35% by volume with the balance of the void space being filled with water and bituminous petroleum. The specific gravity of bituminous petroleum found in tar sand deposits is about 1.0 which further complicates the separation by many processes since bituminous petroleum may be lighter than water or denser than water or they may have essentially the same density.
Methods proposed and evaluated for recovering bituminous petroleum from unconsolidated sand formations includes strip mining and in situ separation processes. Strip mining is feasible only in those deposits located relatively close to the surface of the earth, and in situ separation processes have generally not been technically and/or economically successful. Among the various in situ separation processes described in the literature are thermal techniques such as fire flooding or in situ combustion and steam flooding, as well as emulsification drive processes which may also utilize steam. Solvent flooding is also feasible, but losses of solvent to a formation in a conventional throughput process are high and thus solvent processes have not been economically viable up to the present time.
Besides the usually high viscosity of bituminous petroleum found in tar sand deposits, other problems are encountered in processes for in situ separation of viscous petroleum from the sand grains. If a substantial amount of the sand is produced to the surface of the earth, disposal of the sand becomes a difficult problem. The production of abnormal amounts of sand in conventional well is detrimental to continued production of petroleum therefrom, and sand control methods which are applicable in conventional oil sands are not especially suitable for use in in situ separation processes applied to tar sand deposits because of the high temperatures frequently involved in in situ separations, as well as the fine grain sands generally encountered in tar sand deposits.
It can be seen from the foregoing that there is a substantial need for a method for recovering viscous petroleum from a subterranean, unconsolidated sand or other mineral formation whereby most of the sand is left in the formation and the petroleum is selectively removed from the formation.